1. Field of the Invention
The invention relates generally to the production of aluminum and more specifically to a method and an apparatus for producing aluminum ingots from ore using electrolysis of molten alkali-aluminum sulfate with alkali sulfates at a temperature of approximately 700 degrees Centigrade.
2. Description of the Prior Art
Aluminum smelting is a process characterized by low productivity per unit reactor (around 0.7 metric tons/day), a low thermal efficiency (about ten percent) and a high consumption of electrical energy (about thirteen to seventeen kWh/kg of aluminum). In practice, aluminum is deposited at the cathode with an electrolytic efficiency of 85-90%. All present technologies for aluminum smelting start with alumina (Al.sub.2 O.sub.3) produced and refined by the Bayer process from bauxite using caustic compounds like sodium hydroxide (NaOH). The emphasis in research and development has been directed to the carbo-thermal reduction of alumina to aluminum at about 980 degrees Centigrade by the Hall-Heroult molten salt electrolysis using consumable carbon electrodes, where the alumina is dissolved in cryolite (aluminum-sodium flouride AlNa.sub.3 F.sub.6) and aluminum is deposited electrolytically in molten form. The reactions are EQU 2Al.sub.2 O.sub.3 +3C.fwdarw.3CO.sub.2 +2Al.sub.2 ;
and EQU 2Al.sub.2 O.sub.3 +6C.fwdarw.6CO+2Al.sub.2.
Changing process temperature can adjust the proportion of electrical energy and the proportions of carbon monoxide (CO) and carbon dioxide (CO.sub.2) and thus the amount of carbon required. Modern Hall-Heroult cells operate at four to five volts and a current of about 200,000 amperes.
Scale and process efficiencies of both the Bayer process and the Hall-Heroult cell (both over one hundred years old) have been improved considerably but still warrant further development for better energy efficiency.
Alternative processes include chlorination with electrolysis of aluminum chloride and electrolytic decomposition of alumina using inert electrodes. Future developments rely on advances in material science relating to stable wettable materials for anodes and cathodes. Most likely such advances and process alternatives will be retrofitted to existing Hall-Heroult technology which is likely to dominate aluminum smelting for many more decades.
The goals of future process improvements include:
a) increased thermal efficiency and reduced energy consumption; PA1 b) reduced production and labor cost (inert instead of consumable electrodes); PA1 c) increased production efficiency by continuous instead of batch processing, which also reduces labor cost; PA1 d) extended lifetime of the electrolytic cell; PA1 e) facility to retrofit new developments to existing equipment; and PA1 f) environmental acceptability including avoidance of environmental penalty costs concerning byproducts like gaseous carbon, fluorine and sulfur compounds as well as slag. PA1 A) aluminum sulfate Al.sub.2 (SO.sub.4).sub.3, according to the equation EQU Al.sub.2 (SO.sub.4).sub.3 +K.sub.2 SO.sub.4 .fwdarw.2KAl(SO.sub.4).sub.2 ; and (4) PA1 B) potassium-aluminum sulfate KAl(SO.sub.4).sub.2 directly, as a byproduct of oil production from certain oil sands, oil shales or other aluminum-bearing ores. This process recycles KAl(SO.sub.4).sub.2 but requires removal of excess K.sub.2 SO.sub.4 from the cell.